Across public junior high schools in 21 Colombian cities, we conducted a randomized evaluation of a mandatory six-month internet-based sexual education course. Six months after finishing the course, we find a 0.4 standard deviation improvement in knowledge, a 0.2 standard deviation improvement in attitudes, and a 55% increase in the likelihood of redeeming vouchers for condoms as a result of taking the course. We find no evidence of spillovers to control classrooms within treatment schools, and we find treatment effects are enhanced when a larger share of a student’s friends also takes the course. The low cost of the online course along with the effectiveness we document suggests this technology is a viable alternative for improving sexual education in middle income countries.